Recently, there has existed increasing interest in energy storage technology. Batteries have been widely used as energy sources in portable phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development for them. In this regard, electrochemical devices are the subject of great interest. Particularly, development of rechargeable secondary batteries is the focus of attention. Recently, continuous studies have been performed to develop a novel electrode and battery having an improved level of capacity density and specific energy.
Among the currently used secondary batteries, lithium secondary batteries, developed in early 1990's, have a drive voltage and an energy density higher than those of conventional batteries using aqueous electrolytes (such as Ni-MH batteries, Ni—Cd batteries and H2SO4—Pb batteries), and thus are spotlighted in the field of secondary batteries. However, lithium secondary batteries have a problem related to their safety, due to ignition and explosion caused by the use of a non-aqueous electrolyte. The aforementioned problem becomes more serious as the capacity density of a battery increases.
Meanwhile, in the case of a non-aqueous electrolyte secondary battery, problems related to safety occur under overcharge conditions due to the following reasons: Cathode active materials, such as a lithium-containing metal oxide, capable of lithium and/or lithium ion intercalation/deintercalation are converted into thermally unstable structure due to the deintercalation of lithium under overcharge conditions. Under such overcharged conditions, if the battery reaches a critical temperature due to external physical impact (e.g. exposure to high temperature), oxygen is liberated from the cathode active material having an unstable structure. Then, the oxygen causes an exothermic decomposition reaction with an electrolyte solvent, or the like. Also, combustion of the electrolyte, caused by the above exothermic reaction, is accelerated by the oxygen liberated from the cathode. Such chain reactions accompanied with heat emission finally cause a so-called thermal runaway phenomenon of the battery, resulting in explosion and breakage of the battery.
Many solutions have been suggested in order to control the ignition or explosion of a battery, caused by an increase in the internal temperature of the battery. For example, it is known to use an additive for a non-aqueous electrolyte. Such additives for a non-aqueous electrolyte include an additive based on a reduction-oxidation shuttle reaction, such as chloroanisole, an additive based on a polymerization reaction, such as biphenyl, an alkylbenzene derivative such as cyclohexylbenzene, or the like.
However, the additive based on a reduction-oxidation shuttle reaction is not effective under a high charging current. Additionally, biphenyl has a problem related to the quality of a battery when it is used alone as an additive for a non-aqueous electrolyte, due to an increase in the resistance of the battery. Further, when using an alkylbenzene derivative such as cyclohexylbenzene, there are problems in that a large amount of additive should be added to prevent heat emission caused by overcharge, and prevention of overcharge cannot be accomplished after repeating charge/discharge cycles, resulting in degradation in the quality of a battery.
Therefore, there is a continuous need for a means for improving the safety of a non-aqueous electrolyte secondary battery.